Electron paramagnetic resonance (EPR) spectra are used to characterize transition metals and organic radicals, and interactions among metals or between metals and organic radicals, in biological systems. Quantitative application of EPR techniques for these studies requires knowledge of T(1) of the paramagnetic species. Pulsed EPR and continuous wave (CW) EPR provide measures of very long and very short relaxation times, but there is a "blind" region of at least two orders of magnitude that overlaps the relaxation times of many metals of biological importance. It is proposed to construct an accessory to an EPR spectrometer and develop the methodology to implement a method based on microwave amplitude modulation, which builds upon background experiments in solid state physics. Enhancements that will provide an order of magnitude improvement in sensitivity and about an order of magnitude improvement in ability to measure shorter relaxation time relative to prior efforts include the use of an innovative crossed-loop (bimodal) resonator (CLR), a 200 MHz lock-in amplifier that has recently become commercially available, and the use of a double-balanced mixer (DBM) to provide high-speed modulation of the microwave amplitude. The modulation spectroscopy system will be built as an accessory for a commercial EPR spectrometer. T(1) will be measured with modulation spectroscopy using samples for which long and short T(1) values have been measured by pulsed and CW methods, to test whether the values overlap at the long and short ends of the range for which modulation spectroscopy is expected to be important.